1. Field of the Invention
The present invention relates to a scanning tunneling microscope (hereinafter referred to as "STM") , an information processing apparatus utilizing such a structure, and a displacement element which is used in these devices.
2. Related Background Art
In recent years, research has been conducted about the application of the techniques of an STM to various fields such as the observation/evaluation of semiconductors, polymeric materials or the like in an atomic order or a molecular order, fine processing (E. E. Ehrichs, 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, '89, S13-3) , and recorders.
Above all, in the fields of calculation information and image information and the like of computers, the demand of recorders having a large capacity increases more and more, and with the advancement of semiconductor processing techniques, micro-processors are miniaturized and calculation power is enhanced. Thus, the miniaturization of the recorders is desired.
For the purpose of satisfying the above-mentioned demand, an information processor having a minimum recording area of 10 nm square has been suggested in which a work function on the surface of a recording medium is changed by applying a voltage to the surface of the recording medium from a probe for tunnel current generation provided on a driving means capable of finely adjusting a space between the recording medium and the probe, thereby writing information, while a change of a tunnel current attributed to the change of the work function is detected, thereby reading information.
In this type of processor, it is necessary to scan a sample which is the medium in the range of several nm to several .mu.m by the probe, and in this case, a displacement element using a piezoelectric element is used as a moving mechanism.
FIGS. 22A and 22B show a conventional cantilever type displacement element. FIG. 22B is a sectional view taken along the line 22B--22B' in FIG. 22A.
As shown in FIGS. 22A and 22B, a lower electrode 223 is formed on an elastic member 221, and a piezoelectric layer 222 is then formed thereon. Furthermore, an upper electrode 224 is formed thereon so as to sandwich the piezoelectric layer 222, so that the cantilever can be bent by strain in a direction vertical to an electric field direction, i.e., expansion/contraction defined by a piezoelectric constant d.sub.31, as is widely known in the art.
In addition, recently, an attempt has been made to finely prepare a probe driving mechanism by using a micro-machining technique (K. E. Peterson, Proceedings of the IEEE, Vol. 70, pp 420, 1982) in which a semiconductor processing technique is utilized. FIG. 23 shows an example in which a cantilever type STM probe comprising a piezoelectric bimorph is formed on an Si substrate by the micro-machining technique (T. R. Albrecht, Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, '89, S10-2).
FIG. 23 is the perspective view of the above-mentioned cantilever type STM probe. The cantilever is formed on an Si substrate 231 by laminating a lower electrode 223, a ZnO piezoelectric thin film 235, a medial electrode 233, a ZnO piezoelectric thin film 235 and an upper electrode 224, and a portion of the Si substrate under the cantilever is removed by anisotropic etching so that the cantilever is attached to the edge of the Si substrate to be overhung therefrom. A metal probe 238 is attached to the tip of the cantilever comprising this piezoelectric bimorph by adhesion or another means, and it detects a tunnel current through an outgoing electrode 237. In this case, as shown in a sectional view of FIG. 24, when a voltage is applied under control to two piezoelectric regions sandwiched between the upper electrodes 224 and the medial electrode 233 of the cantilever as well as two piezoelectric regions sandwiched between the lower electrodes 223 and the medial electrode 233, the cantilever can be moved three-dimensionally moved. In this conventional example, Al is used as the electrodes, and so silicon nitride is interposed between ZnO and the electrode. FIGS. 25A, 25B and 25C show the movement of the cantilever at the time when the cantilever is driven by changing the combination of the voltage application regions of the four piezoelectric regions divided by the two-divided electrodes. FIG. 25A shows the movement of the cantilever at the time when an in-phase voltage is applied so that the four regions may simultaneously contract, and the probe moves in a Y direction in the drawing. In FIG. 25B, the two upper and lower regions on the right side stretch and the two upper and lower regions on the left side contract, and in this case, the probe can move in an X direction in the drawing. Furthermore, in FIG. 25C, the right and left regions on the upper side contract and the right and left regions on the lower side stretch, and in this case, the probe can move in a Z direction in the drawing.
The micro-machining technique permits the fine formation of a probe driving mechanism, and also permits the easy production of a plural number of probes which are required to increase the speed of writing and reading of the information of the information processor.
When the piezoelectric constants d in various directions which are the measures of the degree of defromation to the voltage are compared with each other, it can be understood that in typical piezoelectric materials such as ZnO, AlN and PZT, piezoelectric sliding deformation d.sub.15 and d.sub.33 are larger than a piezoelectric lateral effect d.sub.31.
However, the cantilever type displacement element using conventional parallel plate electrodes shown in FIGS. 22A and 22B mainly utilizes the piezoelectric lateral effect d.sub.31. In order to further increase the displacement, it is preferable to utilize d.sub.15 or d.sub.33.
In the case that the elastic member 221 and the piezoelectric layer 222 are different in thermal expansion coefficients, strain occurs in the plane of the cantilever, which impedes an accurate operation.
In the device shown in FIG. 23, in order to decrease the thickness of the respective constitutional layers and to thereby increase the displacement, the thickness and stress of the respective layers must be sufficiently controlled in laminating the electrodes and the piezoelectric thin films, because the strain of the prepared cantilever takes place dependent upon the thickness and stress of the respective layers. In the probe which is displaceable in the three-dimensional direction shown in FIG. 23, the five-layer portion and the three-layer portion are present in terms of layer structure including ZnO and electrode, and when a stress difference occurs between the respective layers, the strain takes place in the plane of the cantilever at times. That is, the mechanical resonance frequencies in the X, Y and Z directions will change owing to the deformation, with the result that a displacing capacity of the displacement element cannot be sufficiently utilized.
Incidentally, the strain in the plane means the maximum displacement of the strain from the cantilever plane.